This invention relates to the transfer of cargo or personnel between two platforms which have relative motion with respect to one another either vertically or laterally, and more particularly to a system used to control the rate of closure, separation and landing site of a carrier for the cargo or personnel transferred between such platforms.
The presence of wind and waves has a degrading effect to the transfer of cargo between a supply vessel and an offshore platform giving rise to the need for precise control of the rate of closure, separation and landing site of the cargo. The carrier for cargo or passengers is subject to displacements due to wind forces when the carrier is raised and lowered by a winch located on the offshore platform. Both fixed and floating platforms are subject to motions due to the force of waves acting on the structure below the platform. The supply vessel is subject to wave forces which cause it to undergo vertical, lateral and angular motions. The supply vessel must be maneuvered to maintain the landing site on the vessel within an area subtended by the boom for a cable extending from the offshore platform. During the transfer operation, the operator for the winch on the offshore platform attempts to control both the rate of closure by the carrier with the vessel and the touchdown site on the vessel through manipulation of the boom and the length of the winch cable. This traditional mode of carrier and/or personnel transfer is satisfactory during calm water and low sea states, but degrades sufficiently as the seas and wind increase to cause a suspension to the transfer operation.
The operator for the winch on the offshore platform is located remotely from the scene of the carrier landing on the vessel. Therefore, the operator cannot adequately compensate for the vertical and lateral motions of the vessel below the platform, particularly when the vessel is subject to increased wave forces degrading the ship's maneuvering capability, thus lessening control of the landing site. Attempts have been made in the past to compensate for the inability of an operator for a winch on a platform to compensate for the vertical and lateral motions of a vessel during a transfer operation through the use of a tether line coupled between the vessel and the platform. The tether line is used to provide a measure, through a suitable transducer system, of the relative motion for control of the winch on the platform. Examples of this form of transfer system are shown in U.S. Pat. Nos. 3,662,991, 3,648,858, 3,675,900, 3,591,022, 2,875,255, 3,189,195 and 3,753,552.
Alternative apparatus for the transfer of cargo between a platform and a vessel includes articulated landing ramps. However, such ramps have met with limited success, particularly in view of the fact that the supply vessel must closely approach the offshore platform with the attendant danger of collision between the supply vessel and the platform. Moreover, an articulated ramp assembly must be provided for each side of the offshore platform in order that transfer operations are conducted at the lee side of the structure. This reduces the severity of the seas at the site of the transfer operations and enables the positioning of the supply vessel in a manner so that the wave forces act in a direction to move the vessel way from the platform structure in the event of a ship malfunction. Devices, such as articulated ramps, become excessively long when used where waves of large amplitude are encountered in order to minimize the angular motion. The ramps are heavy, difficult to engage and expensive.
The present invention is addressed to improvements in a cargo transfer system through the use of a system including a downhaul winch on the vessel and a lifting winch on the platform. Systems of this general type are shown in U.S. Pat. Nos. 3,398,934 and 4,025,055. However, these systems do not provide for a cargo and/or passenger transfer systems at a controlled rate of closure or separation through constant cable tension under the control of an operator at the designated landing site.